Convert BMP to TIFF

This is the first thing to know and the opposite of what a conversion usually does. The BMP writes three bytes per pixel; the TIFF here is handed four channels — red, green, blue and alpha — and writes them uncompressed as well. A 4000 by 3000 image that is about 36 MB as a BMP lands around 48 MB as a TIFF, and a 1920 by 1080 one goes from 6.2 MB to roughly 8.3 MB.

Nothing has been added to the picture. The extra third is an alpha channel that is almost certainly opaque everywhere, written out at full cost because the baseline format has no way to say "this channel is uniform". If a smaller file was the goal rather than the format, this page is the wrong one and PNG is the right one — it is lossless too, and on the flat material bitmaps usually contain it is a fraction of either file. Reading that sentence and leaving is a legitimate outcome of this page.

TIFF is less a format than a filing system for images, and its readers agree on less than the specification suggests. LZW is patent-free and universally supported. Deflate is not universally supported. JPEG-in-TIFF is a different image inside a TIFF wrapper and behaves like one.

Somebody converting to TIFF is almost always feeding something particular — a printer, a scanner’s own software, a document archive with a written specification — and the one variant every such reader accepts is the plain uncompressed baseline. The cost is size, and it is not small; it is deliberately paid here because a file that will not open is worse than a file that is large. If your destination does accept LZW and the size is a problem, compressing the TIFF afterwards in ImageMagick is a one-line job and a decision you are in a position to make and this converter is not.

A print or archive specification talks in dots per inch, and dots per inch is not a property of a pixel grid — it is a number written alongside it that says how large the grid should be on paper. A BMP has a field for it, in pixels per metre; TIFF has a resolution tag; neither is what the picture is made of.

This pipeline hands the TIFF writer the pixels and the dimensions and nothing else, so do not rely on a resolution tag arriving with any particular value. Where the physical size matters, set it in the software that places the image, or check the tag before submitting. The pixel dimensions are the real information and they are exact: 2,480 by 3,508 is an A4 page at 300 dpi whatever any tag claims, and a specification that asks for 300 dpi is asking for those dimensions.

The registry lists print, scanning and archival as the format’s uses, and that is close to the whole of it. Commercial printers specify it because it is the safest thing to hand a RIP. Document-management and records systems specify it because it has been readable since 1986 and will be readable in another forty years. Laboratory and imaging instruments write it because their software always has.

What those have in common is a requirement written down by somebody else. Nobody converts to TIFF for their own convenience — the file is larger, no browser but Safari opens it, and every general-purpose tool prefers something else. That is why this page is short on persuasion and long on what the output will be.

TIFF is one of the few formats here that can hold CMYK, and print specifications routinely ask for it. This conversion cannot produce it: the pipeline decodes to RGB, works in RGB and writes RGB, and there is no setting that changes that.

Nor should a converter pretend otherwise. Turning RGB into CMYK is a colour-managed operation that depends on the press, the paper and the profile the printer uses, and doing it by formula produces a file that is technically CMYK and wrong — usually in the direction of muddy blacks and a colour cast nobody notices until it is printed. If the specification says CMYK, the separation belongs in software that knows those three things, and the printer will normally supply the profile. The format conversion is not the step that is missing.

TIFF supports multiple images in a single file, and scanning workflows lean on that heavily — a twenty-page document is often a twenty-page TIFF. This conversion does not build one. It takes one bitmap and writes one page.

That is a limit worth knowing before converting a folder of scans in the hope of getting a document back. What comes out is a folder of single-page TIFF files with their original names, which is the right input for a system that assembles them and the wrong deliverable if somebody asked for one file. Combining them is a job for ImageMagick or the scanning software that produced the bitmaps, and both will do it in one command — but it is a step, and it is better planned than discovered at the point of submission.

Both formats store pixels without discarding any, so this conversion has no quality setting and nothing to trade. The values in the TIFF are the values that were in the BMP, and converting back would return the same grid again.

The exception is structural rather than visual: a paletted 8-bit bitmap is expanded to full colour on the way in, so the palette does not survive as a palette even though every pixel’s colour does. For a print or archive destination that is the preferred outcome anyway.

TIFF holds EXIF, XMP, IPTC, an ICC profile and GPS data, and archival specifications often require some of those fields to be populated. It is one of the richest metadata containers in this registry.

The registry records no metadata containers for BMP at all, and the pipeline here passes pixels and nothing else. So the TIFF arrives empty: no capture time, no device, no colour profile, no rights statement. If the archive expects those fields, they have to be written afterwards in a tool that edits TIFF tags, and knowing that in advance is cheaper than having a submission rejected.

The registry gives TIFF a browser support value of partial, and Safari is the whole of it — Chrome, Firefox and Edge never adopted the format. It is the reason this site cannot let the browser do the decoding the way it does for a bitmap, and has to carry its own TIFF implementation on both sides of the conversion.

For the reader it means the finished file cannot be checked by dragging it into a tab. Photoshop, Affinity Photo, ImageMagick and any scanning application will open it, and confirming one file in the software that will actually consume it is worth doing before converting the rest. That is not a formality with this format. TIFF readers vary enough that "it opens in Preview" tells you very little about whether a records system will accept it, which is the entire reason the output here is the plainest variant available.

The limit is 100 MB on the file you drop in, which is about 33 megapixels of uncompressed bitmap. Because the TIFF is larger than the BMP, the output can exceed that figure even when the input did not — an 80 MB bitmap produces something over 100 MB, and it is built in the browser’s memory before it reaches your disk.

That makes large scans the case to plan for. Converting a folder works and returns a ZIP with the original names, but a set of high-resolution page scans is one of the few batches on this site large enough to strain a tab. Converting in groups of a few files costs nothing and avoids finding out where the edge is.